AbstractForest ecosystems are crucial to survival on Earth. This article argues that trees and forests are both vital components of a healthy Earth system and productive examples for expanding International Relations’ disciplinary boundaries. The article discusses the forest in three contexts: the global, the (post)colonial, and from the tree itself. From tree planting as a practice of social and environmental justice, to postcolonial and Indigenous science and knowledge, to the mycorrhizal ‘wood wide web’, a focus on trees, forests, and biosphere opens the possibility for a multispecies IR. Through a consideration of trees and forests in law, treaty, culture, and science at the local and global level, this article adds to a growing literature in IR that strives to bring the non-human, more-than-human, or other-than-human creatively and productively into the discipline. Foregrounding the forest's materiality and trees’ symbolic power for human cultures opens important pathways to understanding how the non-human is, and should, alter and affect global politics.